(a) Field of the invention
The present invention is concerned with a method for determining the fiber orientation in a sheet structure made of fibers or containing such fibers, such as a sheet of paper, a cardboard, a sheet of textile or fiberglass, etc. . .
More particularly, the invention is concerned with a method for quantitatively and locally measuring the fiber orientation anisotropy in a fibrous structure, and with a device for carrying out such a method.
(b) Brief description of the prior art
Quantitative measurement of the fiber orientation anisotropy in a fibrous structure is very important in some industries, such as, in particular, the paper making industry, in order to control and possibly adjust the quality of the paper being made, especially its strength, its behaviour to moisture, its resistance to drying stresses and its dimensional stability.
Indeed, it is well known that numerous factors such as parameters of manufacture or designs of machinery may substantially influence the general orientation of the fibers in a fibrous structure being made. In the paper making industry such factors are, by way of non-restrictive examples:
the shape of the lips at the outlet of the head box from which exits the pulp;
the differential velocity between the pulp and the belt on which it is deposited;
the vibration of the belt; and
the subsequent calendering.
Of course, the non-uniform distribution and/or orientation in the machine direction (M.D.) and cross-direction (C.D.) of the fibers in the sheet of paper that is being made because of one or more of the above listed factors, significantly affects the strength and general behaviour of this sheet and makes it of good or poor quality.
Several methods for determining the fibre orientation and, preferably, quantitatively measuring the fibre orientation anisotropy, are known in the art and some of them are commonly used in the industry.
By way of examples, fibre orientation anisotropy in paper can be measured directly by incorporating to the pulp a certain amount of dyed fibres and observing these fibres with a microscope in the finished sheet (see CROSBY, C. M. et al, Tappi, 103-106, March, 1981). However, such a method is combersome and cannot be used easily in a production plant.
Another method of measuring fibre orientation anisotropy is the one known as "zero-span tensile strength test" (see ANCZUROWSKI, E. et al, Pulp and Paper, 112-115, December 1983). This method which basically consists in measuring the force necessary to tear apart a piece of paper hold between two pairs of jaws is known to be very reliable, and is relatively easy to apply in normal paper production by using it on selected samples. On the other hand, this method is destructive and statistical in nature and thus cannot be used to find out how the fibre orientation anisotropy varies from point to point in the sample, and how this variation is related to other physical properties.
Several methods have also been proposed for measuring local variations of fibre orientation anisotropy, based on the analysis of the diffraction and/or scattering patterns of a visible laser light incident on paper (see, RUDSTROW L. et al, Svensk Papperstidnig, 117-121, March 1970). This technique which has been successfully developped to industrial standards, has the advantage of being non-destructive, but its basic principle calls for a very sophisticated data analysis in order to attain the requested level of precision. Moreover the visible light diffraction depends mostly on the surface condition and does not give any indication regarding the average anisotropy through the whole thickness of the sample.
French laid-open patent application No. 2,514,494 MICRAUDEL SARL discloses a method of determining the fibre orientation anisotropy of a fibrous structure, which is also based on the analysis of the diffraction patterns of a polarized laser light traversing the fibrous structure. The optical pattern of the structure, which is so obtained, is recorded and interpreted to determine the actual distribution of the light intensity as a function of the angular position of the plane of polarization of the laser light, or of the structure.
In practice, such an interpretation which is not disclosed is the French laid-open application is very complicated and slow to carry out, thereby making this method not usable on line. Moreover, this method can only be used with thin paper to avoid that too much extinction of the laser light through the paper makes the diffractions pattern difficult or even impossible to record and interpret.
Still another method of measuring paper anisotropy is based on ultrasonic velocity measurements (see FLEISCHMAN, E. H. Ph.D. thesis, The Institute of Paper Chemistry, Lawrence University, 1981, or BAUM A. G. et al, Tappi, vol. 62, No. 5, May, 1979). This technique is interesting in that it can be used in line, but it usually has a spatial resolution around 3 cm and its results are dependent on many mechanical characteristics of the paper sheet (young's modulus, density, etc.)
Last of all, a further method for determining the fibre orientation in paper is disclosed in U.S. Pat. No. 3,807,868 to VALMET OY. According to this method, a polarized laser light beam is directed at right angles to the surface of the paper and the intensity of the reflected light is measured by means of two polarizers. This technique is interesting in that it can be used on line and it gives an index value for the fibre orientation anisotropy. However, it has the drawback of being a surface measurement technique, which gives no indication regarding the average anisotropy through the whole thickness of the sample.